Nuclear magnetic resonance (NMR) equipment, in particular NMR spectrometers and NMR tomographs, require strong magnetic fields, which are often generated by superconducting magnet coils. The superconducting magnet coils must be operated at a cryogenic temperature. So-called pulse tube coolers are often used to maintain the operating temperature over a long period of time.
Pulse tube coolers are based on a periodic pressure fluctuation in a so-called cold head. A control valve connects a high-pressure reservoir and a low-pressure reservoir of a working gas to the cold head in alternation, with the switching frequency of the control valve typically being approximately 1-2 Hz. The pressures in the high-pressure and low-pressure reservoir are maintained by a compressor. The cold head protrudes into the cryostat of the NMR apparatus and is mounted on the cryostat.
The NMR measurements can be disturbed by mechanical vibrations of the NMR apparatus introduced via the cold head.
It has become known from DE 10 2005 035 892 B4 to provide a flexible line segment in the high-pressure line connecting the compressor to a high-pressure reservoir and in the low-pressure line connecting the compressor to a low-pressure reservoir. The pressure reservoirs, the control valve, the cold head and the cryostat together form a vibration-isolated system. The control valve is mechanically rigidly connected to the cryostat. With this design, the transfer of vibrations from the compressor to the cold head and also interference due to the pressure pulses are minimized. Disturbances originating in the control valve, in the connecting line between the control valve and the cold head or in the cold head itself are not isolated from the cryostat with this design.
Low-frequency vibrations can also be caused by the pressure fluctuations in the working gas at a frequency of approximately 1-2 Hz in a connecting line between the control valve and the cold head.
DE 10 2005 004 269 A1 describes a pulse tube cooler system, wherein the control valve is not rigidly connected to the cold head and the cryostat. A connecting line between the control valve and the cold head branches off to two line segments that are combined in such a way that pressure pulses are compensated vectorially. The line segments lead into the cold head on opposite ends, wherein the front parts of the two line segments lie on a line.
High-frequency mechanical vibrations (typically at a few hundred Hz or more) emanate from the control valve of a pulse tube cooler because of the switching mechanics (for example, rotary mechanics with an electric motor). The connecting line between the control valve and the cold head may be designed to be flexible in order not to transfer these vibrations to the cold head. A length of approximately 50 cm or more is usually necessary for this.
If the connecting line leading to the cold head were branched and if the line segments connected to the cold head were designed to be flexible and were arranged in opposition on a line, it would be readily possible to prevent mechanical vibrations on the cold head, but the design size of the branched connecting line (connecting device) would be very expansive and would no longer be compact enough for many applications.